1. Field of the Invention
The invention in general relates to electronic test equipment and more particularly to a test probe for connecting a circuit element to be tested with an oscilloscope or other electronic measurement device.
2. Statement of the Problem
Electronic circuits may be tested by measuring the voltage or other electrical parameter at various circuit nodes. To make such a measurement, the node to be tested must be electrically connected to a test instrument, such as an oscilloscope. The connection is generally made via a test probe. A test probe is essentially an impedance buffer, that is, a circuit with an output having a significantly different impedance than the impedance of its input. Generally, the voltage or other electrical parameter on the output of the test probe follows the voltage or other electrical parameter applied to its input. In addition, the nominal voltage at the input, that is, the voltage when the probe is not being applied to a circuit to be tested, is generally a zero voltage so that the probe does not apply any voltage to the circuit node to which it is applied. Further, the impedance at the input is as high as possible to prevent the test probe and test instrument to which it is connected from drawing significant current or otherwise significantly altering the electrical parameters on the node to be tested. The impedance of the output is generally a standard value, such as 50 ohms, to which the test instrument is designed to couple. Early test probes comprised simple conductors, such as a wire, and a few passive components, such as resistors, to provide an impedance buffer. Such passive test probes are adequate for connecting test equipment to circuits with DC or relatively low frequency electrical cycles. Present-day high frequency circuits require active probes, that is probes with active circuit elements, such as transistors, driven by a probe power source.
To obtain a high input .impedance and low loading of the circuit to be tested, prior art active test probes have generally utilized JFET or MOSFET transistors, which characteristically have a high input impedance. However, such transistors typically also have a high input capacitance and low band width (BW). As the speed of digital circuits has increased, this high input capacitance and low BW characteristic of FETs have created problems. The high input capacitance results in increased loading as the speed of the circuit to be tested increases, and the low BW results in inaccuracies in measurement due to the misrepresentation of rise time and timing in general within fast logic circuits. Thus, some prior art test probe designs have used bipolar transistors to achieve lower input capacitance and higher BW. However, since bipolar transistors typically have low input impedances, prior art test probe designs utilizing bipolar transistors have been unable to achieve input impedances greater than 10 kohms. Thus such probes are not able to effectively test certain families of TTL and CMOS logic that require probes with higher input impedances. Further, the bipolar transistors generally have their collectors connected to VCC, since the collectors must be at a voltage higher than the base and the base must be at a voltage higher than the emitter for the transistor to operate properly. The collectors also tend to have high frequency current spikes typically produced during transistor switching. Such current spikes passing through the VCC common line, to which the collectors are connected, often cause voltage spikes due to the distributed inductance on the VCC common line, which voltage spikes can cause serious circuit malfunctions unless filtering is used. The conventional manner of filtering such spikes has been the use of power supply bypass capacitors between VCC and ground. The length of bypass capacitors look inductive at high frequencies. This inductance, when combined with the output capacitance of the circuit being bypassed, forms a resonant circuit which results in stability problems if the resonant frequency is within the band width of the probe amplifier. As a result, the BW of prior art bipolar test probe circuits have been limited to 1 gigahertz or less and therefore have not been able to be used to test the increasingly fast circuits available today, such as fast ECL logic circuits. In addition, the bipolar circuits generally have nonlinearities in their outputs due to, for example, thermal changes that produce thermal tails. To reduce these nonlinearities, prior art bipolar test probe circuits have used complementary pairs of bipolar transistors; that is, for each npn transistor there is a corresponding circuit with a pnp transistor. This means that twice as many transistors are required for any such bipolar circuits. In instruments where a significant amount of space is available for the circuit, such as preamplifiers for oscilloscopes, some designs have used op amps to remove nonlinearities, though not in combination with bipolar high frequency circuits. However, test probes must generally be designed so that they can effectively be applied by hand to tiny circuit nodes. Thus there is a need for a test probe that has low input capacitance and a wide band width capable of testing today's fast circuits, provides an input resistance greater than 10 kohms, and at the same time is sufficiently compact that it can fit in a hand-sized probe body.
3. Solution to the problem:
The present invention solves the above problems by providing a test probe that utilizes a bipolar input stage comprising a series of three or more bipolar emitter followers, which results in an input resistance of 100 kohms and a input capacitance of only 0.6 picofarads.
The invention also provides a unique design for biasing the bipolar input stage so that destabilizing bypass capacitors are not required. The input of the test probe is AC coupled to the bipolar emitter followers. This allows the transistor bases of the emitter followers to be biased to a negative voltage when the input of the probe is at the nominal zero volts. The negative biasing of the emitter follower transistor bases allows the collectors of the emitter followers to be at ground potential. Since the collectors are at ground, there is no need for capacitors to bypass to ground.
The AC coupling of the emitter follower stage means that DC and low frequency signals will not be passed to that stage. The invention also provides a DC and low frequency impedance buffer path. Preferably this DC and low frequency impedance buffer path is a provided by an op amp that forces the output of the probe to follow the input at DC and low frequencies. The invention thus provides a dual path amplifier, one path amplifying high frequency signals and the other path amplifying DC and low frequency signals.
Preferably, the voltage for biasing the emitter followers is provided by a second op amp.
The invention also provides a circuit to shift the output voltages of the emitter follower stage back to the probe input nominal zero voltage bias level. Preferably, this circuit is a common base bipolar transistor.
Preferably, the output of the DC and low frequency impedance buffer is fed back into the high frequency impedance buffer output through the base of the common base bipolar amplifier.
Preferably the second op amp, i.e the op amp that adjusts the bias of the bipolar stage, is connected to the output of the probe, which contains the DC and low frequencies via the first op amp, and thus also adjusts the bias of AC bipolar stage to follow the DC and low frequencies, thereby preventing DC inaccuracies in the input stage.
The invention also provides protection against being over driven and further protection against current spikes by utilizing a constant current power source to bias the high frequency amplifier.
The invention provides a test probe that not only has high input impedance and low input capacitance, but also has a BW of about 2.5 gigahertz.